I'm thinking, too, of the way Ye Zun's hair went grey in a few seconds due to his power / trauma, so maybe something similar, but because Wang Xiangyang is human his "power" comes from/through the Brush?

Yes! That fits perfectly! Including the way that unlike Wang Xiangyang, it turns out that Ye Zun can control/contain this sudden overflow of power, so he ends up incredibly powerful.